Nondestructive Evaluation for Bridge Management in The Next Century

by Steven B. Chase and Glenn Washer

The Federal Highway Administration (FHWA) is sponsoring a large
program of research and development in new technologies for the
nondestructive evaluation of highway bridges.

The program is currently focusing on developing technologies to
address a wide range of problems. One focus area is the detection
and evaluation of fatigue cracks in steel highway bridges. Another
area is developing technologies for the rapid and quantitative
evaluation of reinforced concrete bridge decks covered with
bituminous concrete. FHWA is also developing new technologies to
enable the evaluation of a bridge in a global sense. In addition,
projects are underway to develop new technologies for the
evaluation of prestressing steel in prestressed concrete, the
evaluation of bridge substructures, and the development of special
tools and methods to evaluate stay cables. And, to integrate
nondestructive evaluation more fully into bridge management
systems, FHWA is studying the development of new models.

This article summarizes FHWA's current research and development
program for the nondestructive evaluation of bridges.

Objectives

The nondestructive evaluation (NDE) program has two main
objectives:

The first objective is to develop new tools and techniques to
solve specific problems. Some examples are locating, quantifying,
and assessing fatigue cracks on steel bridges; quickly,
efficiently, and quantitatively assessing the condition of
reinforced concrete bridge decks even though they are covered with
asphalt; and developing technology to evaluate the 100,000 bridges
for which we do not know how deep the foundation piles extend or
even, in some cases, whether or not there are piles. Any type of
scour or seismic assessment is meaningless without that
information.

Image of a cross section of reinforced concrete slab
produced by ground-penetrating radar -imaging system, showing
individual reinforcing bars and a determination.

The second objective is to develop technologies for the
quantitative assessment of the condition of bridges in support of
bridge management and to investigate how best to incorporate
quantitative condition information into bridge management systems.
Today, the data that feeds all bridge management systems are based
upon visual inspection and subjective condition assessment. We are
developing technologies to quickly, efficiently, and quantitatively
measure global bridge parameters, such as flexibility and
load-carrying capacity.

Background

FHWA's NDE program is focused on the most
important and pressing needs. The specific projects underway are
intended to solve specific problems. An overview of the bridge
inspection and management problem is presented to explain the
priorities we have established for the NDE program.

The National Bridge Inventory contains about 570,000 highway
bridges. If we exclude culverts and tunnels for the moment, the
inventory still includes about 470,000 bridges. The proportions by
superstructure type are shown in figure 1. Steel bridges outnumber
the other types. Steel is followed by concrete, prestressed
concrete, and timber. There are a few other types of bridges, such
as masonry, iron, and aluminum.

Figure 1
Proportion by material type
(excludes culverts and tunnels).

Figure 2 presents data about these bridges by type and age.
There have been two bridge-building booms - one in the
post-depression era and the second as the interstate system was
constructed. We also see that the majority of bridges built prior
to 1970 were steel bridges and that the proportion of prestressed
concrete bridges has been increasing steadily. There has also been
a small, but steady, number of timber bridges built over the
years.

Figure 2
Age Distribution of Bridges
(excludes culverts and tunnels).

Figure 3 begins to show where the most critical problems are. We
can see the proportion by type of all bridges compared to the
proportion by type for structurally deficient bridges. A bridge is
classified as structurally deficient when it has a poor or worse
rating for the condition of the deck, superstructure, or
substructure or when its load-carrying capacity is significantly
below minimum standards. This classification includes the most
serious types of deterioration.

Figure 3
Proportion by material type
(excludes culverts and tunnels).

There are about 110,000 structurally deficient bridges in the
inventory. While steel bridges represent about 40 percent of the
overall bridge total, they include about 60 percent of the
structurally deficient bridges. Reinforced concrete bridges fare
well in the comparison of proportions, and only a relatively small
proportion of the prestressed concrete bridges are structurally
deficient. Timber bridges, while representing only 9 percent of the
entire number of bridges, represent 20 percent of the structurally
deficient bridges. About half of the nation's timber bridges are
classified as structurally deficient.

Taking this analysis one step further and looking at why the
bridges of different types are classified as structurally deficient
leads us to figure 4. The most frequent reason that steel and
timber bridges are classified as structurally deficient is a low
structural adequacy rating. This means the bridge has a very low
load rating. It is also worth noting that more structurally
deficient steel bridges have bad substructures compared to
virtually equal numbers of steel bridges with bad superstructures
or bad decks.

Two of the highest priorities in the NDE program are developing
technologies for steel bridge inspection and new technologies for
bridge deck inspection.

Figure 5 shows the nation's 576,000 bridges, including large
culverts and tunnels, by date of original construction. It has the
same general shape as figure 2. The black line on the chart shows
the percentage of each age group that is classified as structurally
deficient or functionally obsolete. The number of deficient bridges
steadily increases with age, with 80 percent of those bridges built
between 1905 and 1910 classified as deficient. About 1 percent
(5,000 bridges) becomes deficient each year.

Figure 5
Age Distribution of structures.

Today, about 187,000 bridges are classified as deficient. This
figure has been reduced somewhat over the past few years, but only
after federal bridge funding was increased to approximately $3
billion per year. We are currently building or rehabilitating about
10,000 bridges per year. To deal with the backlog of 187,000
deficient bridges; the 5,000 bridges that become deficient each
year; and to ensure that the $3 billion is spent in an optimal
manner, FHWA is mandating the implementation of bridge management
systems.

The essence of a bridge management system is shown in figure 6.
Data are collected from a number of sources (primarily from
periodic bridge inspections) and are transferred to a large
database. A sophisticated analysis of the data is performed. This
generates prioritized lists of candidate projects, optimizes bridge
replacement and maintenance strategies for various available
funding and resource scenarios, predicts the deterioration of
bridges over time, allows managers to evaluate different management
options, and, in general, provides powerful decision-support tools
to help formulate the best program for bridge management. The
systems that have already been implemented have been a tremendous
benefit to decision-makers.

Figure 6
Bridge Management System.

Bridge management systems, however, are still driven by the data
that are collected about the condition of the bridges. No matter
how sophisticated and elaborate the analysis and no matter how
elegant the algorithms employed, in the final analysis, the
recommended decisions cannot be any better than the data upon which
they are based. Today, these data are based almost entirely upon a
visual bridge inspection with condition evaluation determined by
visible indications of deterioration and distress. Deterioration
that does not manifest some visible symptom is not detected or
quantified.

More accurate and more quantitative bridge condition data result
in better decisions and more efficient and optimal allocation of
bridge resources. NDE technology should be used to effectively and
efficiently collect quantitative data about bridge conditions. This
is especially true for certain types of hidden deterioration, such
as corrosion of reinforcement in concrete or cumulative fatigue
loading in steel bridges.

A portable coherent laser radar scanning system is
deployed under a highway bridge. Using the computer controllable
scanning system, bridge engineers are able to measure deflections
of a bridge at hundreds of individual points in a few
minutes.

FHWA's NDE Research Program

Next is a summary of research and development
projects, sponsored by FHWA's Office of Engineering Research and
Development, in nondestructive evaluation for highway bridges.

This project adapts defense technology developed for buried
land-mine detection to the quantitative inspection of bridge decks.
Looking at bridge decks using two different infrared wavelengths
simultaneously overcomes some of the operational problems
(primarily surface emissivity variations) that have been
experienced with the use of infrared thermography for the detection
and quantification of delaminations on bridge decks. A first phase
evaluation on test slabs demonstrated that dual-band infrared
thermography could detect delaminations in both bare concrete and
asphalt-covered concrete and that surface emissivity variations
could be compensated for by the application of image-processing
techniques. A fully operational, mobile infrared imaging system was
delivered to FHWA in February 1996. It is currently undergoing
field testing to more fully evaluate the benefits of dual-band
infrared thermography on actual bridge decks.

This wireless global bridge monitoring system greatly
facilitates the measurement of strains and deformations. Using
digital radio communication technology eliminates the need for long
cables.

Ground-Penetrating Radar Imaging for Bridge Deck
Inspection

This project will develop an engineering prototype of a new
generation ground-penetrating radar for bridge deck inspection. The
system will use impulse radar, synthetic aperture techniques, and
sophisticated signal processing and imaging algorithms to image a
2-meter-wide portion of a bridge deck at one time. The goal is a
system that will travel at traffic speeds, image a lane width of a
bridge, and provide two- and three- dimensional images of the
interior of the bridge deck. Using a small-scale prototype,
preliminary tests have been able to provide images of the interior
of a reinforced-concrete test bed; these images show test voids and
reinforcement. A larger prototype system, capable of inspecting a
two-meter-wide strip of bridge deck, is being assembled and is
scheduled for completion in May 1997. Laboratory and field testing
of this system will begin in the summer of 1997. A portable
hand-held imaging system is also planned.

This project is adapted from a system developed for the National
Aeronautics and Space Administration. It is a portable
laser-scanning system that quickly measures the deflected shape of
a bridge with sub-millimeter accuracy. It also measures the
vibration of the bridge and has the potential to facilitate the
application of modal analysis (a branch of structural analysis that
uses measurements of the vibrations of a bridge with respect to
frequency, amplitude, shape, and decay with time to determine
important structural properties of the bridge and to detect damage)
as a bridge-inspection tool. The system was demonstrated in the
field and was delivered to FHWA in February 1996. The system has
been used to scan large test beams and substructure units, as well
as to measure bridge deflection under controlled loading. It will
undergo further test and evaluation through 1997.

Global Bridge Monitoring With Wireless Transponders

This project is developing a wireless bridge-monitoring system.
It consists of a number of sensor-transponder modules that
communicate via spread-spectrum radio to a local controller. Some
modules measure strain and rotation. To minimize cost and technical
risk, the system development emphasizes the use of off-the-shelf
components developed for the cellular telephone and for automotive
applications. The goal is to develop technology that will make it
possible to instrument a bridge at a dozen locations for a cost of
less than $5,000. A prototype system, consisting of a master
controller and four transponders, has been delivered to FHWA. FHWA
plans to purchase an additional master controller and 12 additional
transponders. The system will undergo field and laboratory testing.
Further development depends on gaining the sponsorship of at least
10 states in a pooled-fund study.

The TRIP steel sensor unit (in foreground) is attached
to a bridge, and the maximum strain experienced by the bridge is
permanently recorded by a change in the magnetic character of the
steel. The sensor is read by the portable instrumentation unit
shown in the background.

An innovative approach to monitoring and measuring large bridges
is being developed under another contract, which was initiated in
October 1995. This project makes the transition from a proven
concept into an operational system prototype. Research sponsored by
the Louisiana and Texas departments of transportation demonstrated
the feasibility of a system to provide a cost-effective means of
performing structural deformation surveys. The objectives are to
produce a system that can be easily configured, installed, and
affordably operated by state and local authorities. The resolution
of the system (sub-centimeter) limits its application to large
bridges. The system has been tested on the Fred Hartman Bridge over
the Houston Ship Channel. (See related article in Public
Roads, Spring 1997 issue, page 39.)

Bridge Overload Measurement and Monitoring Using TRIP Steel
Sensors

A major contributing factor to the deterioration of the nation's
bridges is overload. These overloads are caused by heavy trucks and
earthquakes. To make the most efficient use of bridge inspection
resources, it would be very helpful to have a passive device that
could detect and measure the maximum load experienced by a bridge.
A contract to develop such a system (to continue work initially
sponsored by the Georgia Department of Transportation) was awarded
in November 1995. The system is based upon the use of
transformation-induced plasticity (TRIP) steel sensors. TRIP steel
is a special steel with a special chemical formulation, and it
undergoes a permanent change in crystal structure in proportion to
peak strain. It changes from a non-magnetic to a magnetic steel.
The change can be easily measured. This project will improve the
design and development of these peak strain sensors and will test
their performance on instrumented bridges. These sensors could
provide a reliable, inexpensive, and easily implemented means for
quantitative bridge assessment as a key element of a comprehensive
bridge management system.

Advanced Fatigue-Crack Detection and Evaluation
ProjectsNUMAC

The New Ultrasonic and Magnetic Analyzer for
Cracks (NUMAC), a new fatigue-crack detection system combining
ultrasonic and magnetic inspection capabilities into a single
instrument, has been successfully developed, demonstrated, and
delivered to FHWA. This system consists of a backpack computer and
a head-up display; it features one-hand operation, which is
essential for use on a bridge. This system will greatly improve our
capability to detect and quantify fatigue cracks in steel bridges
even though they may be covered with paint. The prototype system,
received by FHWA, has been loaned to the Colorado and Delaware
departments of transportation for evaluation.

This Small Business Innovative Research (SBIR) project was
initiated in October 1995. It is based on the use of commercially
available high-resolution thermographic imaging systems to detect
surface-breaking fatigue cracks. The method, called forced
diffusion thermography, uses active heating of the bridge surface
with a high-wattage light to detect cracks. A special pattern of
hot and cold regions is created on the steel bridge, and the
thermographic imaging system presents the operator with an image of
heat flow patterns. If a crack is present, a characteristic pattern
is observed. A six-month, first phase study proved the concept by
demonstrating the ability to detect paint-covered fatigue cracks. A
second phase has been initiated to develop a fieldable and
commercially viable system.

Acoustic Emission Monitor for Bridges

FHWA solicited a cooperative agreement in 1995 to work with
industry to co-sponsor the development of an acoustic emission
monitoring system that was specifically engineered and packaged to
meet the need to monitor a fatigue crack on an in-service highway
bridge. FHWA and Physical Acoustics Corp. are sharing the cost of
developing this system. The new system will be small, rugged, and
battery-powered, and it can be left in place for unattended
monitoring for up to one week. It is important to note that this
acoustic emission (AE) system could also be used to determine the
effectiveness of a fatigue crack retrofit. This portable AE system
will be very useful for monitoring and evaluating fatigue
cracks.

Wireless Strain Measurement System

One of the impediments to the measurement of fatigue loading is
the need to install a strain gauge near the fatigue crack and then
to monitor the random variable amplitude strains for a period long
enough to capture the loading spectrum. With traditional strain
gauges, this is difficult because of the need to get to the
locations where fatigue cracks typically form and the need to run
long wires back to a data acquisition system. A portable, rugged,
yet accurate system for measuring strains at inaccessible locations
is needed.

Highway bridges present severe constraints in terms of
access and power. NUMAC provides a portable, yet powerful,
inspection system for the detection of fatigue cracks on steel
bridges. The unit is designed to allow the user to concentrate on
the inspecition task by eliminating most of the burdens associated
with bringing sophisticated computer-based inspection systems to
the field.

The
Small Business Innovative Research Program sponsored the
development of a wireless strain measurement system. This highly
innovative system consists of rugged, battery-powered (solar cells
are optional) radio transponder modules. These modules are able to
accept up to four standard resistive strain gauges with all power
and signal conditioning provided by the transponder. The system
features 16-bit analog-to-digital conversion and an effective
500-hertz (cycles per second) sampling rate. Up to 10 of these
transponders can be used simultaneously. They can be configured to
form local radio telemetry networks with extensive data error
checking and multipath redundancy for very stable and accurate
wireless data transmission. A local transponder is attached to a
personal computer for data acquisition. This system should greatly
facilitate the field measurement of fatigue loads.

Passive Fatigue Load Measurement Device

The wireless strain measurement system is an excellent tool, but
it is somewhat expensive (about $6,000 per transponder) and has a
limited battery life. A totally passive and inexpensive device for
the measurement of fatigue loading is needed. Under a new contract,
initiated in October 1995, a low-cost, passive device to measure
fatigue load will be developed. It is based on the use of two,
precracked fatigue coupons (strips of aluminum) that strain along
with the bridge. The cracks in the two coupons, made of different
grades of aluminum, grow at different rates. Special gauges are
attached to the coupons to accurately measure the lengths of the
cracks. The measurement is made by plugging a crack-length reader
into the device. It is possible to measure fatigue loading by
measuring how much the cracks have grown.

Fatigue Load Measurement Using Electromagnetic Acoustic
Transducers

As part of a congressionally mandated study with the Constructed
Facilities Center at West Virginia University, a device using an
innovative technology to measure the cumulative fatigue loading of
a typical highway bridge is under development. This device has
electromagnetic acoustic transducers that use electromagnetic
fields to generate and detect high-frequency stress waves in steel.
The system can measure the strain in steel members by detecting the
change in travel time of stress waves. The advantages of this
system are that it attaches magnetically to the steel bridge with
very little surface preparation and that dynamic stress
measurements can be taken quickly. In a separate, but concurrent,
developmental effort, Sonic Force Corp. produced a commercial
product based on this technology, and further development of this
system by FHWA is not anticipated.

Eddy Current Detection of Weld Cracks

The goal of this project is to develop a method to detect cracks
in weld metal through bridge coatings. The eddy current method uses
induced magnetic fields to inspect the surface of conductive
materials, such as steel or aluminum. Traditional applications of
this method are not effective in weld metal due to the wide
variation in magnetic material properties. This project is
developing and testing a method using a differential probe that
suppresses these variations in material properties, allowing the
detection of cracks in the weld crown and at the weld toe. The
method has been shown to be effective through both conductive and
nonconductive coatings, including zinc-based primers and lead
paint. Current research is investigating the correlation between
crack signals and crack depth.

Crack Detection Using ACFM

The Alternating Current Field Measurement (ACFM) method, which
is related to the eddy current method, is also being evaluated.
ACFM was originally developed for the offshore oil and gas
industries, where crack detection methods capable of penetrating up
to 5 mm of coating are required. The method uses an induced
magnetic field and a unique probe detection scheme to detect and
quantify longitudinal cracks at the weld toe. The method is
sensitive in a variety of conductive materials, including steel and
aluminum, and can penetrate typical bridge coatings. Current
research is aimed at determining the accuracy of the crack depth
and length measurements and exploring how the method may be used
for the detection of cracks in fillet welds on light poles and sign
supports.

Ultrasonic Time-of-Flight Diffraction

One of the most commonly used inspection techniques for steel
structures is pulse-echo ultrasonics. A sound beam is induced in
the material being inspected, and reflections of that beam are
interpreted to determine the location and size of defects. FHWA is
currently investigating a method know as time-of-flight
diffraction, which uses a pitch-catch transducer configuration to
detect cracks and determine the crack depth with a high degree of
accuracy. The goal of this project is to develop a method for
easily imaging crack profiles in the field, using a specially
designed scanner assembly. This tool will be used to determine the
severity of cracks in steel bridges.

This project is the continuation of the
development of a specialized inspection system for bridge cables. A
prototype system using an array of shuntable, permanent magnets has
been designed and fabricated. This system detects the changes in
the strong magnetic field that is set up by the magnets; these
changes occur if a broken or corroded cable is present. Using
similar technology, commercially available systems can inspect
smaller cables, but typical bridge stay cables are too large. The
system will be upgraded and developed into a dedicated cable-stay
inspection system.

Impact-Echo System for Detection of Voids in Post-Tensioning
Ducts

A recently completed project developed a device for the
detection of voids in the grout on post-tensioned bridges. The
long-term reliability and safety of these bridges depend on the
integrity of the post-tensioning system. This device uses the
impact-echo principle. A known energy pulse strikes a concrete
surface, and the local response is measured using a piezoelectric
transducer. The frequency and energy content of the response can be
used to detect voids in grouted ducts. The system is smaller and
better suited for use on vertical and irregular surfaces than
another impact echo system designed primarily for bridge deck
evaluation. The system is currently on loan to the Maine Department
of Transportation.

Embedded Corrosion Microsensor

In cooperation with the Virginia Transportation Research
Council, an embedded microsensor is being developed to
quantitatively measure corrosion activity inside concrete. The
integrated circuit in an embeddable package will provide
electrochemical measurements of corrosion rate with polarization
resistance and will measure chemical parameters such as
acidity-alkalinity, chloride ion concentration, and temperature.
The sensor will use wireless communications for power and to
telemeter sensor data. The objective is to develop a small and
inexpensive package that will allow hundreds or thousands of
sensors to be embedded in concrete structures. It will then be
possible to quickly scan a concrete structure and to quantitatively
measure the rate and location of corrosion before visible
deterioration has occurred. A prototype integrated circuit has been
fabricated, and further development is ongoing.

A new thermographic imaging system produced this image
of a 1-centimeter-long fatigue crack in a steel specimen. The
remote imaging system was able to detect this crack from a distance
of more that 60 centimeters. The crack was not detected by visual
inspection because it was covered by several layers of
paint.

Magnetic-Based System for NDE of Prestressing Steel in
Prestressed Concrete

This project will develop a portable and versatile inspection
tool for the detection and quantification of corrosion and strand
breakage in prestressed concrete. It is similar to the magnetic
flux leakage inspection system for bridge cables, and it also uses
wireless communications. The system is built around a magnetic
scanning head. The scanning head includes a strong permanent
magnet; a Hall-effect sensor array, which detects changes in
magnetic field strength; a position encoder; and a wireless
communications unit. This portable, self-contained system will be
used to scan prestressed concrete girders and beams and to
telemeter the information back to a portable computer for signal
processing and analysis. The results can be displayed as an image
for rapid anomaly identification. It might also be useful for
inspecting decks, columns, and abutments.

Substructure deterioration is a major reason
for structural deficiency of bridges. There is also a pressing need
to evaluate substructures for scour vulnerability and for
post-earthquake evaluation. An innovative approach for quantitative
substructure evaluation will be tested under a contract awarded in
November 1995. This project will use measured structural movements
caused by induced vibrations to determine the condition of the
substructure. The response will enable engineers to determine the
presence of piles and to establish a base line for subsequent
evaluations. This technology could help evaluate the scour
vulnerability of the approximately 100,000 bridges with unknown
foundations.

An array of eight magnetic sensor modules is
being used to inspect a 25-centimeter-diameter cable for broken
wires. The magnetic flux leakage inspection system uses very strong
permanent magnets to induce a magnetic field in the cable. As the
array is moved along the cable, magnetic sensors detect
characteristic patterns in the magnetic field if a wire is
broken.

Cable-Stay Force Measurement Using Laser
Vibrometers

Dynamic analysis will also be the basis for a new approach to
the quantitative measurement of the forces in stay cables. This
innovative approach will use non-contact laser vibrometers, which
are commercially available, to provide a rapid, low-cost, yet
accurate method for force measurement. The forces in the stay
cables are an excellent indicator of overall structural health for
these types of bridges. The concept was tested on the Stubenville
Bridge in West Virginia in October 1996. The coherent laser-radar
system being developed in a separate project could also be used to
perform dynamic cable-stay measurements.

This first-of-its-kind study will investigate how to develop a
unified quantitative methodology for the integration of
nondestructive bridge evaluation into bridge management systems.
The study will establish relevant measures of damage for bridge
components; it will establish formal links between the results of
NDE measurements and condition states; and it will develop a
methodology for NDE-assisted bridge inspections. The methodology
and procedure will be demonstrated in field inspections on at least
12 highway bridges in six states. Deliverables from this contract
will include complete damage descriptions of all commonly
recognized elements, a complete basis for the translation of NDE
measurements to condition states, and guidance in the application
of NDE-assisted inspections for highway bridges. This will be an
ambitious step for the improvement of bridge management in the next
century.

This study builds on the results of previous research (sponsored
by the Strategic Highway Research Program) in measuring and
predicting the condition of concrete bridge decks. This study will
develop methodologies to improve bridge deck deterioration models
and to incorporate preventative treatments into bridge management
systems, and the study will develop criteria for using
nondestructive testing for these purposes.

This integrated circuit contains all the components
necessary to measure corrosion rates and replaces a bench-top full
of dedicated instruments. This circuit will be integrated with
wireless power and wireless communications circuits to provide a
totally embeddable corrosion sensor.

Exploratory Research ProjectsFundamental Research in Acoustic Emission

FHWA is also funding exploratory research at
the National Institute of Standards and Technology in Boulder,
Colo. This led to the development of an improved, wide-band AE
detector, which will soon be available commercially; a unique AE
laboratory test system that provides the ability to generate and
detect fatigue cracks in different extension modes; and advanced
finite element modeling of acoustic emission generation,
propagation, and detection.

Fundamental Magnetostrictive Sensor Research

FHWA is funding fundamental research in the development of
sensors based upon magnetostriction. A magnetic field produces a
small change in the physical dimensions of ferromagnetic materials.
By coupling a coil of wire with a bias magnet, a useful sensor can
be constructed. Such senors would be low-cost, simple, and rugged.
Possible applications include detection and measurement of
corrosion and breakage in prestressing strands, monitoring the
curing of concrete, and as embedded acoustic emission sensors.

Use of Microwaves to Detect and Quantify Fatigue
Cracks

Another topic of FHWA-funded exploratory research is the
development of small microwave waveguide sensors. If a microwave
waveguide is placed against a steel plate, it is effectively
short-circuited, and a characteristic standing wave is created.
This standing wave can be detected with a very inexpensive diode.
If a fatigue crack is present in the plate, the standing wave
changes. A very rapid, yet low-cost, fatigue-crack detector can be
produced. Current studies focus on the evaluation of lift-off (the
distance from the surface being tested to the sensor) and the
detection of crack edges. A paint coating acts as a spacer between
the steel surface and the sensor; the thicker the paint, the
greater the lift-off, and the less sensitive the sensor
becomes.

Fiber-Optic Strain Sensor

FHWA and the Naval Research Laboratory are sponsoring
development of a fiber-optic strain sensor. This project has
demonstrated that it is feasible to use the Bragg grating
interferometric method, which measures the changes in the frequency
of reflected light, to measure strains in concrete bridge beams. A
prototype system that could measure strain at up to 16 locations
simultaneously is being developed.

Laboratory Support Contract to Assist in Testing,
Evaluating, and Validating NDE TechnologiesLaboratory Support Contract for Test and Evaluation of New
Technologies

As can be seen from the wide array of new technologies being
studied and under development, the testing and evaluation of these
systems will be a big job. Recognizing this, FHWA awarded a
contract in October 1995 to provide technical, professional, and
logistical support to the Special Projects and Engineering Division
for the testing and evaluation of these new systems.

Nondestructive Evaluation Validation Center

In addition to the testing and evaluation that will be conducted
in accordance with the laboratory support contract, FHWA needed
better facilities and capabilities to evaluate and validate the new
nondestructive evaluation technologies and systems being developed
by FHWA and others. Special funding was provided in fiscal year
1996 to support the design and construction of a new Nondestructive
Evaluation Validation Center at the Turner-Fairbank Highway
Research Center (TFHRC) in McLean, Va. A contract was awarded to
Wiss, Janey, Elstner Associates Inc. in September 1996 to design,
construct, and operate this center. The center will renovate the
existing small-structures laboratory at TFHRC to provide a modern
and fully equipped NDE testing facility. In addition, three highway
bridges within 250 kilometers of TFHRC will be made available for
full- scale testing of NDE technologies under actual field
conditions. The center will also acquire a wide variety of
specimens from highway bridges that contain typical defects, which
will be fully characterized. "Fully characterized" means that all
aspects and properties of the bridge have been measured and
quantified to the maximum extent possible. These specimens, ranging
from steel elements with fatigue cracks to full- scale girders and
decks, will be maintained in a library of specimens, which will be
used by FHWA and other researchers and developers to test and
validate existing and new NDE technologies. One of the first
studies to be undertaken once the center is operational is a
probability of detection study of visual inspection of highway
bridges.

Conclusion

FHWA's program of research and development in
nondestructive evaluation is helping to ensure the safety of the
nation's highway bridges. The program is also providing more
accurate and complete information about the condition of the
nation's highway bridges. Many of the research and development
projects produce prototypes of new inspection systems. Much of
FHWA's future activities in the NDE program will involve extensive
testing of these prototypes, both in the field and at the new NDE
Validation Center. We plan to work with our customers, bridge
owners throughout the United States, to conduct much of this
testing. These tests and experiments will fully assess and validate
the capabilities of new and existing inspection systems and
methods, define the limits of the technologies, identify any needed
iterative development, and define directions for future research.
This approach ensures that follow-on implementation and
demonstration projects are a logical continuation of the technology
development process and involve technologies that are fully
understood and are ready for widespread deployment.

Dr. Steven B. Chase is a research structural
engineer in the Structures Division of FHWA's Office of Engineering
Research and Development at the Turner-Fairbank Highway Research
Center in McLean, Va. He is the program manager for FHWA's
Nondestructive Evaluation Research and Development Program.

He joined FHWA in 1978 as a highway engineer trainee with a
bachelor's degree in civil engineering. He earned both his master's
degree in civil engineering in 1984 and a doctorate in civil and
environmental engineering in 1991 through the FHWA Academic Study
Program. He worked as an FHWA bridge engineer in Washington, D.C.;
in FHWA's regional Office of Structures in Fort Worth, Texas; and
in the division offices in Massachusetts and Rhode Island. He
joined the staff of the Structures Division in 1992.

Glenn Washer is a research engineer in the
Special Projects and Engineering Division of FHWA's Office of
Engineering R&D at TFHRC. He is the special projects manager
for FHWA's NDE R&D Program. He has a bachelor's degree in civil
engineering from Worcester Polytechnic Institute and a master's
degree in civil engineering from the University of Maryland. He is
a licensed professional engineer in Virginia.